There are a lot of gaps in United's flight numbers. An example would be : UA 1,2,3,4,7,10,15,16,19,22,39,42,43,44,45,46,47,48,49,50,51... and so on. Now I just made up those numbers, but my point is that there are open numbers, and those will be filled in. Also there are no 29XX flight numbers. Those will most likly become the Metro Jet flights.